I 22F Found a picture of girl in my boyfriends 25M phone

My boyfriend (25M) of four years and I (22F) were looking through some of our photos together (on his phone) and I saw this random snapchat screenshot of a girl in almost seethrough shorts and shirt. It could be seen as just a random mirror selfie, but it’s very close to a photo in lingerie. He swears he doesn’t know why it’s there or who it is, but he agrees it looks very bad and understands why I would immediately assume he’s cheating. He said a pocture like that looks a little sexual to him as in not something you casually send your friends (especially not to your friends who are in a relationship). I don’t know what to do or how to feel. He almost had a panic attack because he feels so bad and is extremely frustrated that he can’t explain the screenshot to me. He deleted it immediately and magically it is also deleted from the “recently deleted” folder….. (he says he didn’t delete it from there). We searched his phone for any hint we could get as to where this picture comes from but there is no trace of it anywhere. No conversations around it. No hints as to what it could be. I feel like I’ve seen the photo before, but I don’t know if I’m just gaslighting myself and building false memories.
I would love to believe him, but I’m very sensitive to this an absolutely terrified at the thought of getting cheated on. Thing is, we had arguments about him potentially emotionally cheating in the past…. He had trouble setting a clear line between me and some female friends: stupid jokes like “if you don’t go out on a date with me I will ask a friend” or when I would ask what he was doing he’d joke about cheating which very early on I told him to stop doing and he did. But also not correcting his boss when he thought another girl was his gf then arguing with me about why that’s okay and why it doesn’t matter if his boss knows. Lying about having a higher body count than he actually did. Lying about having more experience than he has. His friend keeps saying he’s very good at lying and I thought he was just being petty, but now… Idk
Wanting to go out at 2AM to hang out with a girl friend to smoke (I do really believe his intentions were pure bit he took a loooong time to understand why all this bothered me).
I have to say I am also very insecure about this stuff. I really don’t know what to do, feel or think.

I hope this is detailed enough, feel free to ask questions I might have forgotten something.

I just want to be at peace. Do you have any advice on what to do? I don’t know how to handle this and I’m afraid I can’t take it for much longer… he already has a lot of other stuff to work on in our relationship

TL;DR
My bf had a snapchat screenshot of a random girl on his phone, no trace of any other contact with her. Maybe a friend sent it but there is no proof of it. Idk what to do….